In the early days of electrical power, direct and alternating voltage power systems competed in the marketplace. Due to the ability of magnetic transformers to adjust voltage up and down, and also possibly as a result of the forces of advertising, the use of alternating voltage for powering homes and factories superseded the use of direct voltage. Early alternating-voltage (also known as alternating current or AC) systems were somewhat dangerous to the public, because of the possibility the case of equipment connected to the power line could become energized in the event of an internal fault. In such a situation, the ground or earth became part of the electrical path, and someone who inadvertently came into contact with the case of the equipment could experience the full alternating mains voltage. Many deaths occurred due to this effect.
Within the last fifty years or so, the use of “three-wire” or grounded systems has provided a large measure of protection against electrocutions due to undesired faults between the AC power lines and ground. The third wire grounded the case of the equipment and opened protective devices in the power distribution system, preventing the case from becoming energized. Somewhat later, “double insulated” products were introduced, which had a lesser likelihood of allowing personal contact with either side of the AC power line. Within the last twenty years or so, “ground fault interrupters” have been widely used, and in many cases mandated, for use in kitchens, bathrooms, and exterior power outlets. These interrupters monitor current flowing from the power distribution system to ground, and in the presence of ground currents deemed to be significant, which may be on the order of a few milliamperes, disconnect the load from the power source.
When alternating current (AC) became dominant for powering houses and factories, direct current systems continued to be used for certain purposes. These purposes included the powering of flashlights and other light duty applications by means of batteries, and the powering of ancillary systems such as lights, controls, and communications in mobile systems such pleasure boats, automobiles, and airplanes. For the most part, these battery-operated direct-voltage systems tended to have relatively low voltages. For example, an ordinary flashlight might have two series-connected 1½ volt batteries, for a total of 3 volts. Automobile accessories such as lamps-and engine self-starters were originally powered by six-volt batteries, which were supplanted by 12-volt batteries. Such voltages are low enough so that dry skin provides protection against the flow of significant current, to the extent that a person may not notice application of 12 volts to the body.
With the advent of modern electrically powered vehicles, the use of high power direct voltages has once again become important. The direct voltage powering arises because of the need to store electrical energy for mobile use, which at the current state of the art requires battery operation. Both all-electric and hybrid-electric vehicles use direct-voltage traction batteries to power traction motors to drive the vehicle.
Because of the large amounts of energy required to propel an automobile, the traction battery of an electrically powered vehicle must be suited to the provision of substantial energy, for at least a short time. Those skilled in the art know that a traction battery must have a relatively large storage capacity, and must deliver a relatively large amount of power, compared to a conventional 12 volt automobile storage battery. It is further understood that because power is directly proportional to battery voltage and system current, the high power delivery requirements which must be satisfied by traction batteries necessarily mean that higher electrical voltages (to keep current levels within a practical range) will be present in electric automobiles than in automobiles powered by fossil-fuel internal-combustion engines, which typically require only a comparatively low power, low voltage storage battery for energizing auxiliary loads when the internal combustion engine is not operating.
Hybrid electric vehicles (HEVs) combine the internal combustion engine of a conventional vehicle with the battery and electric motor of an electric vehicle. This results in an increase in fuel economy over conventional vehicles. The combination also offers extended range and rapid refueling that users expect from a conventional vehicle, with at least some of the energy and environmental benefits of an all-electric vehicle. The practical benefits of HEVs include improved fuel economy and lower emissions compared to conventional vehicles. The inherent flexibility of HEVs also permits their use in a wide range of applications, from personal transportation to commercial hauling.
Electric or hybrid-electric vehicles require less combustion of fossil fuels by comparison with conventional internal-combustion engines. Such vehicles are becoming increasingly attractive alternatives to fossil fuel powered cars. However, because of the high voltage requirements of its traction battery, an electric or hybrid electric vehicle raises significant electrical reliability and safety concerns.
It should be understood that the flow of electrical current is always in a loop, with the electrons leaving one terminal of the system (as for example a “+” battery terminal) and flowing to another terminal (the corresponding “−” terminal). Mere contact with a single terminal is insufficient, in itself, to establish a complete loop path which would allow the flow of electricity. In order to help in preventing unwanted flow of electrical energy in an electrical vehicle context, the traction battery and motor are often operated in an electrically “floating” or ungrounded mode, in which the traction battery and motor equipments are totally isolated from the housing or chassis in which it is located, so that the traction power system electrical current flows in a closed loop. This has a safety advantage, because a person who inadvertently comes into contact with a terminal of the traction power system is not in danger of experiencing electrical shock due to current flow from the traction power system to chassis. In addition to promoting safety, such isolated operation of the traction power system promotes reliability by tending to prevent undesired current flow in sensitive control and ancillary electrical circuits. However, there is always the possibility of formation of an unwanted path for the flow of electrical current or “ground fault” between the nominally isolated traction power system and the chassis, due to moisture, damage, corrosion, or the like. Such a ground fault gives rise to the possibility of current flow through portions of the chassis and sensitive equipment. A more important potential effect of such a ground fault is to place a person in danger of electrical shock should they simultaneously come into inadvertent contact with a terminal of the traction power system and the vehicle chassis, as the traction power system with a ground fault is no longer isolated from chassis. The high voltages and high electrical current capability of a traction power system make such shock potentially lethal. Additionally development of a second ground fault could cause large, potentially destructive currents to flow.
Various systems have been proposed for detecting ground faults in a direct-voltage context. U.S. Pat. No. 5,481,194 issued Jan. 2, 1996 in the name of Schantz et al. describes a system that uses a resistive voltage divider to produce, at its tap or node, a voltage “centered” between the positive and negative direct-voltage buses of the traction power system. The voltage at this node is compared with a reference voltage, and the voltage difference is amplified and compared with a threshold to thereby declare a ground fault if the voltage difference exceeds a particular value. U.S. Pat. No. 5,561,380 issued Oct. 1, 1996 in the name of Sway-Tin et al. includes positive and negative sampling circuits which, in the absence of a ground fault, produce equal-amplitude voltages. In the presence of a ground fault, the sampled voltages become unequal or unbalanced. A ground fault is declared when the inequality exceeds a threshold which varies with the battery voltage. Another ground fault detection scheme is described in an article entitled DC Leakage Current Detector Protects the High Voltage Equipment User by Pete Lefferson, published at pages 34–37 of the September 2000 issue of Power Conversion and Intelligent Motion (PCIM). The Lefferson arrangement connects a resistance-capacitance network to a balance point of the direct voltage source, and the voltage across the capacitor is monitored and compared with a threshold. A ground fault is declared when the capacitor voltage exceeds a threshold. Another ground fault detector arrangement is described in U.S. Pat. No. 6,678,132, issued Jan. 13, 2004 in the name of Carruthers et al. In this arrangement, active centering of the balance voltage occurs, and a ground fault is declared when the unbalance exceeds the range of the active centering control.
Improved or alternative ground fault detection arrangements are desired.